Transport of alpha-ketoisocaproate in rat cerebral cortical neurons

Transport of alpha-ketoisocaproic acid (KIC), the product of leucine transa mination, was studied in the cerebral cortex cells isolated from the adult rat brain. The process of [C-14]KIC accumulation was followed in the presen ce of aminooxyacetate, an inhibitor of transaminases. Accumulation of KIC w as not affected by Na+ replacement, its initial velocity was observed to be higher upon lowering of external pH. Addition of KIC promoted acidificatio n of cytoplasmic pH, monitored with 2'7'-bis(carboxyethyl)-5(6)-carboxyfluo rescein, The detected inhibition of KIC accumulation by alpha-cyano-4(OH)ci nnamate pointed to an involvement of one of monocarboxylate transporters (M CT), although 4,4'-diisothiocyano-2,2'-stilbenedisulphonate was without eff ect. Accumulation of KIC was inhibited by lactate; the effect of pyruvate w as detected to be much weaker. Other branched-chain alpha-ketoacids (ketois ovalerate, keto-methylvalerate), as well as beta-hydroxybutyrate and valpro ate decreased the transport of KIC by 30, 60, and 80%, respectively. The ob served characteristics of KIC accumulation in the cortical neurons indicate an involvement of one of the MCT transporters. A crucial role of SH group( s) in the process of KIC accumulation, excluding MCT2, indicates the MCT1, although an involvement of another isoform of MCT in the process of KIC tra nsport in neurons from cerebral cortex of adult brain has not been definite ly excluded. (C) 2000 Academic Press.